It turns out that editing is not as smooth as I want it. TaskManager and ResourceMonitor revealed that the CPU was mostly bored at 12% usage, memory not really used much, but at regular intervals there were large disk access rates in the vicinity of 40MB/sec, as if PD was prereading large file chunks.
So, if I believe these performance measurements, memory or graphic card upgrades don't seem to be the right strategy for improving the editing experience (I don't know if I can upgrade the CPU and have no plan to do that for now).
I am therefore thinking to add an SSD, but how can I convince PD to use it? I am also confused by conflicting opinions in this forum, e.g. "there is no noticeable increase in speed in video editing" in this thread. Therefore, I would be grateful for advice how to make my editing more enjoyable. Thinkpad E540, 16GB memory, Core i3 4000M
HP workstation Z420, Xeon E5-1620, 32GB, Quadro 600